This invention pertains to a method and an apparatus for removal of particulates from oil in a vacuum pump, as used to maintain a partial vacuum in a diffusion furnace for chemical vapor deposition in intermittent runs wherein oil for the vacuum pump becomes contaminated by particulates. In chemical vapor deposition of thin films of various materials on various substrates in a diffusion furnace, in which a partial vacuum is maintained by an oil-sealed mechanical vacuum pump, it is common for by-products to be formed as particulates, which contaminate oil for the vacuum pump, and which must be removed at least intermittently from oil for the vacuum pump so as to avert damage to the vacuum pump and/or loss of partial pressure.
Chemical vapor deposition of thin films of silicon nitride, Si.sub.3 N.sub.4, is used for its masking, passivating, and insulating capabilities, particularly for its resistance to diffusion of ionic contaminants. Silicon nitride, Si.sub.3 N.sub.4, may be deposited in various reactions of various reactants, as exemplified by silane, SiH.sub.4, and ammonia, NH.sub.3, and by silicon tetrachloride. SiCl.sub.4, and ammonia, NH.sub.3. Pertinent references include W. A. Kohler, "Structural Properties of Vapor Deposited Silicon Nitride," Metallurgical Transactions, Vol. 1, March 1970, pages 735 through 740, and R. Ginsburgh et al., "Silicon Nitride Chemical Vapor Deposition in a Hot-Wall Diffusion System," Journal of the Electrochemical Society, Vol. 125, No. 9, September 1978, pages 1557 through 1559.
As an example of particular interest, it is known to deposit thin films of silicon nitride, Si.sub.3 N.sub.4, on silicon substrates by chemical vapor deposition in a diffusion furnace, in which a partial vacuum is maintained by an oil-sealed mechanical vacuum pump, which is maintained at an interior temperature of about 800.degree. C., and into which reactive gases--three standard parts of ammonia, NH.sub.3, and one standard part of dichlorosilane, SiH.sub.2 Cl.sub.2 --are metered, whereupon ammonium chloride, NH.sub.4 Cl, is produced as a by-product, and as particulates, which contaminate oil for the vacuum pump, and which must be removed at least intermittently from oil for the vacuum pump so as to avert damage to the vacuum pump and/or loss of partial pressure. Pertinent references include R. S. Rosler, "Low Pressure CVD Production Processes for Poly, Nitride, and Oxide," Solid State Technology, April 1977, at pages 63 through 70, and H. Kurokawa et al., "Low-Pressure CVD Films Growth Technique and its Equipment," IEEE Denshi Tokyo, 1977, pages 86 through 88.
It is known to filter particulates from oil for a vacuum pump, as used to maintain a partial vacuum in a diffusion furnace for chemical vapor deposition as discussed above, in microporous filters having replaceable cartridges, through which oil for the vacuum pump is circulated as the vacuum pump is operated. Such filters tend to become clogged rapidly, whereupon the vacuum pump must be shut down frequently, so as to permit clogged cartridges to be changed. Usage of oil under these conditions is high.
Experimental attempts have been made to remove particulates from oil for a vacuum pump, as used as in the example discussed above, by withdrawing oil from the vacuum pump to a settling tank, pumping oil from the settling tank, filtering particulates from oil pumped from the settling tank in a series of microporous filters having replaceable cartridges, and returning oil to the vacuum pump after passage through the cartridges, for timed intervals, wherein each interval has been initiated upon a timed measurement that accumulated running time of a predetermined duration had elapsed in serial runs, which tended to vary in duration from run to run. Such attempts have not been consistently satisfactory, as the cartridges nonetheless tended to become clogged excessively in some runs, sometimes before the runs had terminated.
A medium to high vacuum from about 450.times.10.sup.-3 Torr to about 5.times.10.sup.-3 Torr is maintained in a diffusion furnace for chemical vapor deposition as discussed above. Various types of oil-sealed mechanical vacuum pumps, as may be used to maintain such a vacuum, are discussed in standard references including L. I. Maissel et al., Eds., Handbook of Thin Film Technology, McGraw-Hill Book Company, New York, 1970, under "Vacuum Pumps" at pages 2-4 through 2-17, and T. Baumeister et al., Eds., Marks' Standard Handbook for Mechanical Enginners, 8th Ed., McGraw-Hill Book Company, New York, 1970, under "High Vacuum Pumps" at pages 14-45 through 14-48.
Conventional settling and filtering techniques for removal of particulates from oil are discussed in standard references including J. J. O'Connor, Ed., Standard Handbook of Lubrication Enginnering, McGraw-Hill Book Company, New York, 1968, under "Lubricant Reclamation and Conditioning" at pages 30--1 through 30-3, under "Oil Purifiers" at pages 25-34 through 25-38, and under "Pumps" at pages 25-38 and 25-39, and C. A. Bailey et al., The Lubrication Engineers Manual, U.S. Steel Corporation, 1971, under "Oil Contaminants and Their Removal" at pages 12 through 14.
Herein, "oil" refers to any natural or synthetic oil which may be used in a vacuum pump as discussed above, and which may become contaminated by particulates as discussed above.